The bacterial endotoxin, lipopolysaccharide (LPS), induces expression of multiple early response genes in human monocytes, including the proinflammatory cytokines TNF-a, IL-1b and IL-6. IL-10 expression, which is also LPS-inducible in monocytes, is delayed relative to that of TNF-a, IL-1b and IL-6. IL-10 feedback inhibits expression of these cytokines, as well as IL-10 itself, thereby providing an efficient mechanism for controlling cytokine production in monocytes. The Th1-type lymphokine, interferon-g (IFN-g), markedly upregulates TNF-a production in human monocytes. In this project, we are investigating the effects of IFN-g on IL-10 expression in LPS-stimulated monocytes. We are also examining the relationship between between IL-10 and TNF-a levels in IFN-g-primed cells. LPS stimulation induces rapid and ordered expression of cytokine mRNA. Steady-state mRNA levels for TNF-a increase rapidly, reach maximum levels by 2-3 hr post stimulation, and then decline sharply. IL-1b and IL-6 mRNA levels also increase markedly following stimulation with LPS, but decrease more slowly than TNF-a. Downregulation of mRNA for TNF-a, IL-1b and IL-6 coincides with a delayed amd more gradual increase in IL- 10 mRNA levels. Furthermore, neutralization of endogenous IL-10 with anti-IL-10 antibody prolongs TNF-a mRNA expression, and significantly increases TNF production. IFN-g inhibited expression of IL-10 mRNA in LPS-stimulated monocytes, and decreased net IL-10 production in a dose- dependent manner. The reduction in IL-10 mRNA levels was associated with a marked increase in both the magnitude and duration of TNF-a mRNA expression. Thus, potentiation of TNF-a production in IFN-g-primed monocytes is coupled to inhibition of endogenous IL-10 expression. In future experiments, we will attempt to define the molecular mechanism by which IL-10 downregulates cytokine production, particularly TNF-a, in LPS-stimulated monocytes.